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Himalayan earthquake confirms worst fears over dam

2 November 1991

By FRED PEARCE

Seismologists have called for a halt to construction of a giant dam
in the Himalayas, 45 kilometres from the epicentre of the earthquake that
struck the region last week. The quake, which killed more than a thousand
people, occurred just east of Uttarkashi, upstream of the dam site at Tehri
on the Bhagirathi river, a tributary of the Ganges.

James Brune, professor of geophysics at the University of Nevada, last
week visited Tehri and warned that the dam was ‘one of the most hazardous
in the world from an earthquake point of view’ (see ‘The dam that should
not be built’, New Scientist, 26 January). This week, he said: ‘The earthquake
shows that our concerns were right. At the very least the Indians must now
halt construction for a year or so while they reassess the risks.’

The epicentre of the quake was in the middle of the Central Seismic
Gap that extends for 700 kilometres along the boundary between the Eurasian
and Indian continental plates. A seismic gap is a zone along a plate boundary
where stress is building up because of an absence of large earthquakes in
the recent past. The gap runs between Kashmir, scene of a quake measuring
8.6 on the Richter scale in 1905, and Bihar, where an 8.4 quake struck in
1934.

Brune warns that last week’s movement, which measured 7.1, was too small
to relieve a significant amount of strain along the gap. ‘It will still
be stored up for the big one, which could come at any time.’ Brune believes
that a quake measuring more than 8 (almost ten times the strength of last
week’s event) will be necessary to release the strain, which is caused by
the Indian plate pushing its way beneath the Eurasian plate. He does not
believe that the 260-metre-high dam, now in the early stages of construction
in a gorge on the Bhagirathi, could stand the impact of such a quake.

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‘There is extreme danger not only from the major thrust fault, but also
from numerous branching thrust faults which might rupture the crust much
nearer the dam than the main fault itself, possibly producing peaks of ground
acceleration even greater than produced by the rupture of the main thrust,’
says Brune. One of the most important questions for seismologists investigating
last week’s quake, he says, is whether the rupture extended along these
branch faults close to Tehri.

Brune’s case that the region may still be waiting for ‘the big one’
is supported by leading Indian seismologists such as Vinod Gaur, former
director of the National Geophysical Research Institute. But, says Brune,
‘the engineers who are advising the Indian government do not believe in
the idea of seismic gaps.’

Brune warns that the greatest risk of ‘the big one’ will be in the next
few months, long before the dam can be completed. But even after such a
cataclysm – which could kill tens of thousands of people in Uttarkashi,
Tehri and the surrounding villages – risks would be little diminished. He
estimates that it would take three major quakes to release the tension that
has been building up along the seismic gap.

If the dam is built an earthquake could create a breach in the dam or
cause large landslides that would set up tidal waves in its 45-kilometre
reservoir, which could themselves destroy the dam. Last week there were
reports of landslides close to the area where the planned reservoir will
form. Water released by a breach in the Tehri Dam would rush down the narrow
valley, killing thousands in towns such as Haridwar and the religious centre
of Rishikesh.

Almost a year ago the Indian supreme court rejected an application from
residents at Tehri, a town of 50 000 people. They were demanding that work
on the dam should be halted because the government had failed to ensure
that it would be safe from earthquakes.